2.8x ADD-ON - I need an add-on to "paint" instances of an object over another object's surface
If I understand what your goal is correctly, then here is a solution that should work for you in version 2.8 without needing any add-ons or extra tools. 

Move your clone object(s) to a new collection with M and give it(them) a useful name, such as "hair". 

Add a new vertex group to your main object, Suzanne in our example here. You can leave it at the default name "group" or rename it to something more useful like "hair".

Add a new particle system and change the following settings to begin with:



Most default settings are usually fine, but here you want to Render as Collection, and set the Density Vertex Group

With all of that done, you can now paint your clones. Ctrl+Tab into Weight Paint Mode  and  paint away! Everything will paint into the last active vertex group, adding to the density group of your hair system.Ctrl+Tab again to return to object mode

See the gif below adding icospheres to Suzanne.



You may also randomize velocity if you'd like, (this will allow some randomness in rotations)  the option is pictured below.